In recent years, the prevalence of digital cameras has been significant. With a compact digital camera for beginners to photography, it is desirable to be able to take photos of roughly a certain quality regardless of the user's skill. Therefore, focusing is normally performed automatically, and there is a need to shorten the focusing operation period as much as possible so that a photo opportunity is not missed.
Many compact digital cameras do not have an optical viewfinder, and in the focusing operation period as well, an imaging subject continues to be viewed on a liquid crystal display unit arranged on the back face of the digital camera body. Therefore, in the focusing operation period as well, it is necessary to display an image of the imaging subject on the display unit without causing a sense of unpleasantness.
There are mainly two automatic focusing operation methods, namely an active method and a passive method. An active method is a method in which the distance to an imaging subject is measured by irradiating an imaging subject with infrared radiation or ultrasonic waves and receiving infrared radiation or ultrasonic waves that have reflected off of the imaging subject. Active methods often are employed in compact silver salt cameras.
On the other hand, a passive method is a method in which the distance to an imaging subject is measured based on images captured by an optical system. Passive methods further can be divided into phase difference detection methods and contrast detection methods. A phase difference detection method often is employed in single lens reflexive cameras, regardless of whether the camera is a silver salt camera or digital camera. On the other hand, a contrast detection method often is employed in compact digital cameras.
In a contrast detection method, images are obtained from an imaging element while gradually moving a focus lens in an optical system in the optical axis direction, and the focal position is set to the position of the focus lens where the contrast of the obtained image is maximal. Contrast generally is evaluated based on a high frequency component in an image obtained from an imaging element.
Since the evaluation of the high frequency component is performed on a frame-by-frame basis, shortening the focusing operation period requires raising the read frame rate of the imaging element. However, if the read frame rate of the imaging element is raised, the exposure time becomes shorter. If the exposure time in the imaging element becomes shorter, the image displayed on the display unit becomes darker, and also an automatic focusing operation by the contrast detection method becomes unstable, which is a problem.
A digital camera has been proposed in relation to the above problem (see Patent Document 1). In the digital camera according to Patent Document 1, the read frame rate of an imaging element is increased if the luminance of an imaging subject is greater than or equal to a first threshold, and the read frame rate of the imaging element is decreased if the luminance of the imaging subject is less than or equal to a second threshold.
According to this structure, a high-speed focusing operation is performed with use of a high read frame rate when the imaging subject is bright, and the exposure time is lengthened with use of a low read frame rate when the imaging subject is dark.    Patent Document 1: JP 2003-262788A